Various oxazolidinone derivatives having antimicrobial activity were known in the art. For example, it has been known that (S)-n-[[3-[3-fluoro-4-(4-morpholinyl)phenyl]-2-oxo-5-oxazolidinyl]methyl]acetamide (“linezolid”) has potent antimicrobial activity on methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) (e.g., see Patent Document 1), and it has been approved and commercially available as anti-VRE infection drug.
Additionally, oxazolidinone type antimicrobial agents having a structure as represented by the following formulae, wherein a 5-6 fused ring is connected to the benzene moiety of linezolid, have been reported (Patent Documents 2 to 7 and Non-Patent Documents 1 and 2).    WO2006/035283 (Patent Document 2):
    WO2005/019214 (Patent Document 3):
wherein X is N or C;    WO99/10342 (Patent Document 4):
    WO93/09103 (Patent Document 5):
    WO2002/064574 (Patent Document 6):
                wherein Y is        
    WO2006/109156 (Patent Document 7)
                wherein A is selected from        

WO2008/021781 (Patent Document 8, kinase inhibitor), U.S. Application Publication 2006/0035898 (Patent Document 9, kinase inhibitor), WO2003/053975 (Patent Document 10, PDE7 inhibitor), Japanese Patent Publication 08-151578 (Patent Document 11, compound for liquid-crystal) disclose oxazolidinone derivatives having a fused ring. However, they are not an antimicrobial agent.
Non-Patent Documents 3 and 4 disclose other oxazolidinone type antimicrobial agents.
Patent Document Nos. 12 and 13 and Non-Patent Document 5 disclose antimicrobial agents having a fused hetero ring moiety such as imidazopyridine.
Patent Document 14 discloses a compound having a triazolemethyl group at the 5-position of oxazolidinone ring as an antimicrobial agent.    [Patent Document 1] WO95/07271    [Patent Document 2] WO2006/035283    [Patent Document 3] WO2005/019214    [Patent Document 4] WO99/10342    [Patent Document 5] WO93/09103    [Patent Document 6] WO2002/064574    [Patent Document 7] WO2006/109156    [Patent Document 8] WO2008/021781    [Patent Document 9] U.S. Application Publication 2006/0035898    [Patent Document 10] WO2003/053975    [Patent Document 11] Japanese Patent Publication 08-151578    [Patent Document 12] WO2005/089763    [Patent Document 13] WO2006/038116    [Patent Document 14] WO2001/081350    [Non-Patent Document 1] Zhongguo Yaowu Huaxue Zazhi (2004), 14 (5), p. 263-266    [Non-Patent Document 2] Bioorganic & Medicinal Chemistry (2004), 12 (17), p. 4557-4564    [Non-Patent Document 3] Bioorganic & Medicinal Chemistry Letters 17 (2007) 5036-5040    [Non-Patent Document 4] Journal of Medicinal Chemistry, 2007, Vol. 50, No 20, 4868-4881    [Non-Patent Document 5] Bioorganic & Medicinal Chemistry Letters 19 (2009) 894-899